I don't think Puff can shield grab if the Fair is well spaced. I also don't see how Marth wouldn't be able to juggle Jigglypuff, when she doesn't have good moves from above and her fall speed is extremely slow.
Doesn't punish as hard in Melee? I think her punishing in Brawl has a really bad reward (little damage and not much follow-ups) while when Marth punishes he can either deal 15-20% damage from a DB (a chance of juggle set up is often available too), deal 13% damage from a DS (which also kills at high %s) or set up frame traps from a grab.
I haven't played any Puffs as Marth, but it seems like Puff only has a chance if she somehow gets a lead, because Marth, walls her extremely well (if he can shut down almost every approach Wario has, then he can also definitely wall Jiggs too), though that seems to be very hard for her because Marth punishes harder and it's easier for Marth to punish Jiggs than viceversa.